Dental restorations, such as crowns, bridges and the like, are typically made of a metallic or porcelain core framework with one or more porcelain layers coated thereon. The porcelain layers provide strength, wear resistance and favorable aesthetics to the dental restorations. In porcelain fused-to-metal (PFM) restorations, it is important that the firing temperature of the porcelain be at least 100.degree. C. below the solidus temperature of the alloy used as the metal framework and that the coefficient of thermal expansion of the porcelain (in the range of room temperature to 450.degree. C.) be only very slightly less than that of the metal so that no stress cracks are produced in the porcelain layer during firing and cooling down. Ceramic cores are advantageous in the fabrication of dental restorations because concerns regarding effective concealment of the metal color do not exist. Similar to metal cores, when a ceramic core is used, any porcelain applied to the ceramic framework must possess a coefficient of thermal expansion that is slightly less than that of the ceramic to prevent failure in the porcelain due to stresses caused by thermal expansion mismatch.
U.S. Pat. No. 5,679,144 is directed to a powder composition comprising one or more feldspar frits and CeO.sub.2 as an opacifier. The frits have a particle size distribution of d.sub.50 of 3 to 6 microns, d.sub.90 of 12 to 16 microns and a maximum grain size of less than 20 microns. As set forth therein, since the paste may segregate, it is supplied in ajar or box so that it can be stirred up before use if necessary. Also, due to the relatively fine particle size, pinholes or steam tearing may occur in the final restoration if moisture is not allowed to evaporate freely.
U.S. Pat. No. 4,557,691 is directed to a porcelain paste comprising a porcelain powder mixed with an aqueous colloidal dispersion of a urethane polymer. The product is sold in a syringe and may dry out in the syringe before use due to evaporation of ammonia and water.
U.S. Pat. No. 4,806,383 is directed to a method of applying opaque dental ceramic material to a metal structure that requires an additional step of dusting a ceramic powder having an average grain size of from 40 microns to 100 microns onto the applied opaque paste, firing the material, and applying additional ceramic and firing same. The large particle size affects the handling characteristics of the paste and also results in significant settling of the powder in the liquid. In addition, the dusting step requires additional time and skill to make the restoration.
There is a need to prevent settling of the powder from the liquid component in porcelain paste compositions so that the paste composition can be provided in a variety of packaging forms including syringes. It is desirable to produce a dental porcelain paste that can be applied easily in thin coats. It is advantageous that sufficient coverage of the core framework is achieved without the need for dusting of additional porcelain powder thereon.